Multi-dimensional modeling of mixture preparation in a direct injection engine fueled with gaseous hydrogen

With the recent advances of direct injection (DI) technology, introducing hydrogen into the combustion chamber through DI is being considered as a viable approach to circumvent backfire and pre-ignition encountered in early generations of hydrogen engines. As part of a broader vision to develop a robust numerical model to study hydrogen spark ignition (SI) combustion in internal combustion (IC) engines, the present numerical investigation focuses on mixture preparation in a hydrogen DI SI engine. This study is carried out with a single hole injector with gaseous hydrogen injected at 100 bar injection pressure. Simulations are carried out for high and low tumble configurations and validated against optical data acquired from planar laser induced fluorescence (PLIF) measurements. Varying mesh configurations are investigated for the impact on in-cylinder mixture distribution. A particular emphasis is placed on the effect of nozzle geometry and mesh orientation near the wall. Overall, the computational model is found to predict the mixture distribution in the combustion cylinder reasonably well. The results showed that the alignment of mesh with the flow direction is important to achieve good agreement between numerical analysis and optical measurement data.     

Energy and exergy analysis of a combined injection engine using gasoline port injection coupled with gasoline or hydrogen direct injection under lean-burn conditions

Hydrogen is considered to be a suitable supplementary fuel for Spark Ignition (SI) engines. The energy and exergy analysis of engines is important to provide theoretical fundaments for the improvement of energy and exergy efficiency. However, few studies on the energy and exergy balance of the engine working under Hydrogen Direct Injection (HDI) plus Gasoline Port Injection (GPI) mode under lean-burn conditions are reported. In this paper, the effects of two different modes on the energy and exergy balance of a SI engine working under lean-burn conditions are presented. Two different modes (GPI + GDI and GPI + HDI), five gasoline and hydrogen direct injection fractions (0, 5%, 10%, 15%, 20%), and five excess air ratios (1, 1.1, 1.2, 1.3, 1.4) are studied. The results show that the cooling water takes the 39.40% of the fuel energy on average under GPI + GDI mode under lean-burn conditions, and the value is 40.70% for GPI + HDI mode. The exergy destruction occupies the 56.12% of the fuel exergy on average under GPI + GDI mode under lean-burn conditions, and the value is 54.89% for GPI + HDI mode. The brake thermal efficiency and exergy efficiency of the engine can be improved by 0.29% and 0.31% at the excess air ratio of 1.1 under GPI + GDI mode on average, and the average values are 0.56% and 0.71% for GPI + HDI mode.     

Direct injection of hydrogen main fuel and diesel pilot fuel in a retrofitted single-cylinder compression ignition engine

Up to 90% hydrogen energy fraction was achieved in a hydrogen diesel dual-fuel direct injection (H2DDI) light-duty single-cylinder compression ignition engine. An automotive-size inline single-cylinder diesel engine was modified to install an additional hydrogen direct injector. The engine was operated at a constant speed of 2000 revolutions per minute and fixed combustion phasing of ?10 crank angle degrees before top dead centre (°CA bTDC) while evaluating the power output, efficiency, combustion and engine-out emissions. A parametric study was conducted at an intermediate load with 20–90% hydrogen energy fraction and 180-0 °CA bTDC injection timing. High indicated mean effective pressure (IMEP) of up to 943 kPa and 57.2% indicated efficiency was achieved at 90% hydrogen energy fraction, at the expense of NO_x emissions. The hydrogen injection timing directly controls the mixture condition and combustion mode. Early hydrogen injection timings exhibited premixed combustion behaviour while late injection timings produced mixing-controlled combustion, with an intermediate point reached at 40 °CA bTDC hydrogen injection timing. At 90% hydrogen energy fraction, the earlier injection timing leads to higher IMEP/efficiency but the NO_x increase is inevitable due to enhanced premixed combustion. To keep the NO_x increase minimal and achieve the same combustion phasing of a diesel baseline, the 40 °CA bTDC hydrogen injection timing shows the best performance at which 85.9% CO₂ reduction and 13.3% IMEP/efficiency increase are achieved.     

Effect of fuel injection timing and injection pressure on performance in a hydrogen direct injection engine

The in-cylinder hydrogen fuel injection method (diesel engine) induces air during the intake stroke and injects hydrogen gas directly into the cylinder during the compression stroke. Fundamentally, because hydrogen gas does not exist in the intake pipe, backfire, which is the most significant challenge to increasing the torque of the hydrogen port fuel injection engine, does not occur. In this study, using the gasoline fuel injector of a gasoline direct-injection engine for passenger vehicles, hydrogen fuel was injected at high pressures of 5 MPa and 7 MPa into the cylinder, and the effects of the fuel injection timing, including the injection pressure on the output performance and efficiency of the engine, were investigated. Strategies for maximizing engine output performance were analyzed.The fuel injection timing was retarded from before top dead center (BTDC) 350 crank angle degrees (CAD) toward top dead center (TDC). The minimum increase in the best torque ignition timing improved, and the efficiency and excess air ratio increased, resulting in an increase in torque and decrease in NO_x emissions. However, the retardation of the fuel injection timing is limited by an increase in the in-cylinder pressure. By increasing the fuel injection pressure, the torque performance can be improved by further retarding the fuel injection timing or increasing the fuel injection period. The maximum torque of 142.7 Nm is achieved when burning under rich conditions at the stoichiometric air-fuel ratio.     

Effects of split injection proportion and the second injection timings on the combustion and emissions of a dual fuel SI engine with split hydrogen direct injection

In this paper, a new kind of injection mode, split hydrogen direct injection, was presented for a dual fuel SI engine. Six different first injection proportions (IP1) and five different second injection timings were applied at the condition of excess air ratio of 1, first injection timing of 300°CA BTDC, low speed, low load conditions and the Minimum spark advance for Best Torque (MBT) on a dual fuel SI engine with hydrogen direct injection (HDI) plus port fuel injection (PFI). The result showed that, split hydrogen direct injection can achieve a higher brake thermal efficiency and lower emissions compared with single HDI. In comparison with single HDI, the split hydrogen direct injection can form a controlled stratified condition of hydrogen which could make the combustion more complete and faster. By adding an early spray to form a more homogeneous mixture, the split hydrogen direct injection not only can help to form a flame kernel to make the combustion stable, but also can speed up the combustion rate through the whole combustion process, which can improve the brake thermal efficiency. By split hydrogen direct injection, the torque reaches the highest when the first injection proportion is 33%, which improves by 1.13% on average than that of single HDI. With the delay of second injection timing, the torque increases first and then decreases. With the increase of first injection proportion, the best second injection timing is advanced. Furthermore, by forming a more homogeneous mixture, the split hydrogen direct injection can reduce the quenching distance to reduce the HC emission and reduce the maximum temperature to reduce the NO_X. The split hydrogen direct injection can reduce the HC emission by 35.8%, the NO_X emissions by 7.3% than that of single HDI.     

Hydrogen release from a high pressure gaseous hydrogen reservoir in case of a small leak

The dynamic blow-down process of a high pressure gaseous hydrogen (GH₂) reservoir in case of a small leak is a complex process involving a chain of distinct flow regimes and gas states. This paper presents models to predict the hydrogen concentration and velocity field in the vicinity of a postulated small leak. An isentropic expansion model with a real gas equation of state for normal hydrogen is used to calculate the time dependent gas state in the reservoir and at the leak. The subsequent gas expansion to 0.1 MPa is predicted with a zero-dimensional model. The gas conditions after expansion serve as input to a newly developed integral model for a round free turbulent H₂-jet into ambient air. Predictions are made for the blow-down of hydrogen reservoirs with 10, 30 and 100 MPa initial pressure. A normalized hydrogen concentration field in the free jet is presented which allows for a given leak scenario the prediction of the axial and radial range of flammable H₂-air mixtures.     

Analysis of the particulate emissions and combustion performance of a direct injection spark ignition engine using hydrogen and gasoline mixtures

Three different fractions (2%, 5%, and 10% of stoichiometric, or 2.38%, 5.92%, and 11.73% by energy fraction) of hydrogen were aspirated into a gasoline direct injection engine under two different load conditions. The base fuel was 65% iso-octane, and 35% toluene by volume fraction. Ignition sweeps were conducted for each operation point. The pressure traces were recorded for further analysis, and the particulate emission size distributions were measured using a Cambustion DMS500. The results indicated a more stable and faster combustion as more hydrogen was blended. Meanwhile, a substantial reduction in particulate emissions was found at the low load condition (more than 95% reduction either in terms of number concentration or mass concentration when blending 10% hydrogen). Some variation in the results occurred at the high load condition, but the particulate emissions were reduced in most cases, especially for nucleation mode particulate matter. Retarding the ignition timing generally reduced the particulate emissions. An engine model was constructed using the Ricardo WAVE package to assist in understanding the data. The simulation reported a higher residual gas fraction at low load, which explained the higher level of cycle-by-cycle variation at the low load.     

Research on the hydrogen injection strategy of a turbulent jet ignition (TJI) rotary engine fueled with natural gas/hydrogen blends

Natural gas/hydrogen blends (NGHB) fuel is considered as one of the ideal alternative fuels for the rotary engine (RE), which can effectively reduce the carbon emissions of RE. Additionally, applying turbulent jet ignition (TJI) mode to RE can significantly increase the combustion rate. The purpose of this study is to numerically investigate the influence of hydrogen injection position (HIP) and hydrogen injection timing (HIT) on the in-cylinder mixture formation, flame propagation and NOx emission of a TJI hydrogen direct injection plus natural gas port injection RE. Therefore, in this paper, a test bench and a 3D dynamic simulation model of the turbulent jet ignition rotary engine (TJI-RE) fueled with NGHB were respectively established. Moreover, the reliability of the 3D simulation model was verified by experimental data. Furthermore, based on the established 3D model, the fuel distribution and flame propagation in the cylinder under different HIPs and HITs were calculated. The results indicated that the HIP and HIT could change the hydrogen distribution by altering the impact position, impact angle, and the strength of vortexes in the cylinder. To improve the flame propagation speed, more hydrogen should be distributed in the pre-chamber. Additionally, a higher concentration of hydrogen in the cylinder should be maintained above the jet orifice. This was not only conducive to the rapid formation of the initial fire core in the pre-chamber, but also significantly improved the combustion rate of the in-cylinder mixture. Compared with other hydrogen injection strategies, the hydrogen injection strategy by using the HIP at the middle of the cylinder block and the HIT of 190^oCA(BTDC) could obtained the highest peak value of in-cylinder pressure and the highest NOx emission.     

Comparison of performance and emissions of a supercharged dual-fuel engine fueled by hydrogen and hydrogen-containing gaseous fuels

This study investigated the engine performance and emissions of a supercharged engine fueled by hydrogen (H₂), and three other hydrogen-containing gaseous fuels such as primary fuels, and diesel as pilot fuel in dual-fuel mode. The energy share of primary fuels was about 90% or more, and the rest of the energy was supplied by diesel fuel. The hydrogen-containing fuels tested in this study were 13.7% H₂-content producer gas, 20% H₂-content producer gas and 56.8% H₂-content coke oven gas (COG). Experiments were carried out at a constant pilot injection pressure and pilot quantity for different fuel-air equivalence ratios and at various injection timings. The experimental strategy was to optimize the pilot injection timing to maximize engine power at different fuel-air equivalence ratios without knocking and within the limit of the maximum cylinder pressure. Better thermal efficiency was obtained with the increase in H₂ content in the fuels, and neat H₂ as a primary fuel produced the highest thermal efficiency. The fuel-air equivalence ratio was decreased with the increase in H₂ content in the fuels to avoid knocking. Thus, neat H₂-operation produced less maximum power than other fuels, because of much leaner operations. Two-stage combustion was obtained; this is an indicator of maximum power output conditions and a precursor of knocking combustion. The emissions of CO and HC with neat H₂-operation were 98-99.9% and NOx about 85-90% less than other fuels.     

Numerical simulation of the mixture distribution and its influence on the performance of a hydrogen direct injection engine under an ultra-lean mixture condition

In this study, a three-dimensional numerical model of a hydrogen direct-injection engine was established, and the combustion model was verified by experimental data. The influence of the injection timing and nozzle diameter on ultra-lean combustion was evaluated. The results suggest that, with the delay in the injection timing, the mixture concentration near the spark plug and combustion speed gradually increase. The maximum thermal efficiency increased from 47.44% to 49.87%. The combustion duration and ignition lag are shortened from 19.15°CA to 11.15°CA to 16.13°CA and 5.92°CA, respectively. As the nozzle diameter increased, the injection duration was shortened, and the mixture distribution area became more concentrated. Furthermore, under ultra-lean combustion, the combustion rate is more sensitive to the distribution of the mixture. Appropriately increasing the equivalence ratio near the spark plug can significantly shorten the ignition lag and combustion duration and obtain a higher thermal efficiency.     

Experimental study of the polytropic index of the compression stroke for a direct injection hydrogen engine

Hydrogen is regarded as a promising alternative fuel in the future. The direct-injection (DI) hydrogen engine has been demonstrated to offer large power without the risk of abnormal combustion. For the engine test of combustion analysis, the piezoelectric transducer measures the dynamic cylinder pressure data rather than the absolute one. The traditional method of absolute cylinder pressure correction is normally based on the polytropic index. This paper investigated the polytropic index effects by injection duration, start timing of injection and speed based on a 2.0 L direct-injection hydrogen engine. The experiments found that, since the injection of large volume hydrogen leads to approximately 0.8 bar increase of cylinder pressure in the compression stroke, the traditional correct method is not suitable for the DI hydrogen engine. What's more, the polytropic index drops from 2.2 to 1.22 with the changes of the crank angle and is sensitive to the concentration of the hydrogen. The study of the instantaneous polytropic index can not only be used to calculate the accurate heat release rate but also guide the research of the heat transfer of the DI hydrogen engine. The average polytropic index of 1.32 provides a new reference for the absolute cylinder pressure correction of the DI hydrogen engine.     

Operation principles for hydrogen spark ignited direct injection engines for passenger car applications

The sustainable reduction of greenhouse gas emissions from road transport requires solutions to achieve net-zero carbon dioxide emissions. Therefore, in addition to vehicles with electrified powertrains, such as those implemented in battery electric of fuel cell vehicles, internal combustion engines fueled with e-fuels or biofuels are also under discussion. An e-fuel that has come into focus recently, is hydrogen due to its potential to achieve zero tank-to-wheel and well-to-wheel carbon dioxide emissions when the electrolysis is powered by electricity from renewable sources. Due to the high laminar burning velocity, hydrogen has the potential for engine operation with high cylinder charge dilution by e.g. external exhaust gas recirculation or enleanment, resulting in increased efficiency. On the other hand, the high burning velocity and high adiabatic flame temperatures pose a challenge for engine cooling due to increased heat losses compared to conventional fuels. To further evaluate the use of hydrogen for small passenger car engines, a series production 1 L 3 cylinder gasoline engine provided by Ford Werke GmbH was modified for hydrogen direct injection. The engine was equipped with a high pressure external exhaust gas recirculation system to investigate charge dilution at stoichiometric operation. Due to limitations of the turbocharging system, very lean operation, which can achieve nitrogen oxides raw emissions below 10 ppm, was limited to part load operation below BMEP = 8 bar. Thus, a reduction of the nitrogen oxides emission level at high loads compared to stoichiometric operation was not possible. At stoichiometric operation with external exhaust gas recirculation engine efficiency can be increased significantly. The comparison of stoichiometric hydrogen and gasoline operation shows a reduced indicated efficiency with hydrogen with significant faster combustion of hydrogen at comparable centers of combustion. However, higher boost pressures would allow to achieve even higher indicated efficiencies by charge dilution compared to gasoline engine operation.     

Development of a large-sized direct injection hydrogen engine for a stationary power generator

A number of studies on hydrogen engines have targeted small-sized engines for passenger vehicles. By contrast, the present study focuses on a large-sized engine for a stationary power generator. The objective of this study is to simultaneously achieve low NOx emission without aftertreatment, and high thermal efficiency and torque. Experimental analysis has been conducted on a single-cylinder test engine equipped with a gas injector for direct hydrogen injection. The injection strategy adopted in this study aims generating inhomogeneity of hydrogen mixtures within the engine cylinder by setting the injection pressure at a relatively low level while injecting hydrogen through small orifices. High levels of EGR and increased intake boost pressures are also adopted to reduce NOx emission and enhance torque. The results showed that extreme levels of EGR and air-fuel inhomogeneity can suppress NOx emission and the occurrence of abnormal combustion with little negative impact on the efficiency of hydrogen combustion. The maximum IMEP achieved under these conditions is 1.46 MPa (135 Nm@1000 rpm) with engine-out NOx emission of less than 150 ppm (ISNOx < 0.55 g/kW) for an intake boost pressure of 175 kPa and EGR rate of around 50%. To achieve further improvement of the IMEP and thermal efficiency, the Atkinson/Miller cycle was attempted by increasing the expansion ratio and retarding the intake valve closing time of the engine. The test engine used in this study finally achieved an IMEP of 1.64 MPa (150 Nm@1000 rpm) with less than 100 ppm of NOx emission (ISNOx < 0.36 g/kWh) and more than 50% of ITE.     

船用直喷式天然气发动机喷射策略的优化

利用CONVERGE软件基于L23/30DF型船用天然气发动机建立了双天然气喷嘴、双引燃柴油喷嘴的直喷天然气发动机的缸内燃烧过程的CFD计算模型,计算了不同的柴油和天然气喷射时刻和间隔下发动机缸内燃烧和排放过程.结果 表明:引燃柴油的喷射时刻及其与天然气喷射时刻的间隔,对直喷式天然气发动机燃烧和排放性能有重要影响.当喷...     

Experimental study on lean-burn characteristics of an SI engine with hydrogen/gasoline combined injection and EGR

Hydrogen has shown potential for improving the combustion and emission characteristics of the spark ignition (SI) dual-fuel engine. To reduce the additional NO_x emissions caused by hydrogen direct injection, in this research, the cooperative control of the addition of hydrogen with exhaust gas recirculation (EGR) in the hydrogen/gasoline combined injection engine was investigated. The results indicate that both the addition of hydrogen and the use of EGR can increase the brake mean effective pressure (BMEP). As the α_H2 value increases from 0% to 25%, the maximum BMEP increases by 9%, 12.70%, 16.50%, 11.30%, and 8.20%, respectively, compared with the value without EGR at λ = 1.2. The CA0-10 tends to increase with increases in the EGR rate. However, the effect of EGR in increasing the CA0-10 can be offset by the addition of 15% hydrogen at λ = 1.2. Measurements of the coefficient of variation of the indicated mean effective pressure (COV_IMEP) indicate that the addition of hydrogen can effectively extend the EGR limit. Regarding gaseous emissions, NO_x emissions, after the introduction of EGR and the addition of hydrogen, are lower than those of pure gasoline without EGR. An 18% EGR rate yields a significant reduction in NO_x, reaching maximum decreases of about 82.7%, 77.8%, and 60% compared to values without EGR at λ = 1.0, 1.2, and 1.4, respectively. As the EGR rate increases, the hydrocarbon (HC) emissions continuously increase, whereas a blend of 5% hydrogen can significantly reduce the HC emissions at high EGR rates at λ = 1.4. Finally, according to combustion and emissions, the coupling of a 25% addition of hydrogen with 30% EGR at λ = 1.2, and the coupling of a 20% addition of hydrogen with an 18% EGR rate at λ = 1.4 yield the best results.     

Visualization of hydrogen jet evolution and combustion under simulated direct-injection compression-ignition engine conditions

The evolution and combustion of H₂ jets were investigated in an optically-accessible constant-volume chamber under simulated direct-injection (DI) compression-ignition (CI) engine conditions. The parameters varied include injection pressure (84–140 bar) and ambient temperature (1000–1140 K). A detailed characterization of the injector system and the ensuing jet penetration process is reported first. High-speed schlieren imaging, OH1 chemiluminescence imaging and pressure trace measurements were subsequently used to investigate the auto-ignition and combustion of the H₂ jets. The results show that the ignition delay of H₂ jets under such conditions is sensitive to ambient temperature variations, but not to injection pressure. Optical imaging reveals that the combustion of H₂ jets mostly initiated from a localized kernel, before spreading to engulf the whole jet volume downstream of ignition location. The imaging also indicates that after ignition, the flame recesses back towards the nozzle and appears to attach to the nozzle to form a diffusion flame structure.     

A review of ammonia as a compression ignition engine fuel

During the past decades, the diesel engine has been through times of upheaval, boom and bust. At the beginning of the century, almost 50% of the new vehicle registrations in the European market were diesel-powered. However, the news of deadly diesel NO_x emissions supported by the diesel emission scandals caused a shock to the diesel engine market, and the sustainability of the diesel engine is currently in dispute.Recently major automotive manufacturers announced the development of diesel-powered vehicles with negligible NO_x emissions. Moreover, the NO_x emissions production is of lower concern for heavy-duty, marine or power generations applications where the implementation of advanced aftertreatment systems is feasible. However, despite the tackle of NO_x emissions, the decarbonisation of the automotive, marine and power generation markets is mandatory for meeting greenhouse gas emissions targets and limiting global warming.The decarbonisation of the diesel engine can be achieved by the implementation of a carbon-free fuel such as ammonia. This paper provides a detailed overview of ammonia as a fuel for compression ignition engines. Ammonia can be combusted with diesel or any other lower autoignition temperature fuel in dual-fuel mode and lead to a significant reduction of carbon-based emissions. The development of advanced injection strategies can contribute to enhanced performance and overall emissions improvement. However, ammonia dual-fuel combustion currently suffers from relatively high unburned ammonia and NO_x emissions because of the fuel-bound nitrogen. Therefore, the implementation of aftertreatment systems is required. Hence, ammonia as a compression ignition fuel can be currently seen as a feasible solution only for marine, power generation and possibly heavy-duty applications where no significant space constraints exist.